1. Field of the Invention
The invention relates to a copper strip or sheet with a red-brown to dark brown cover layer for use in the construction sector. The invention is also directed to preferred methods for the production of a brown cover layer on strip-shaped semi-finished products consisting of copper, particularly on rolled strips and sheets for roofing and facade paneling.
2. Description of Related Art
It is known that under normal atmospheric conditions, a firmly adhering and strong cover layer of copper oxide forms on the surface of shiny copper. The oxide film, even though it is very thin at first, stabilizes the surface of the copper material right away, protecting it from the effects of the atmosphere. In an ideal case, the slow continued formation of the oxide layer as the result of the reaction of the copper with moisture and oxygen in the air gradually causes a uniform brown coloration (brown patina) to be formed, with the copper surface increasingly losing its metallic shine. Over time, the brown cover layer becomes darker and darker, and finally changes to an anthracite-brown. This is the final state which usually occurs on vertical building surfaces, such as exterior wall paneling. In the case of inclined roof surfaces, the cover layer continues to change color, reacting with the substances contained in the atmosphere, such as sulfur dioxide, carbon dioxide and chlorides, to form alkaline copper compounds, until the patina green which is typical for copper is reached.
However, under certain atmospheric conditions, the formation of the brown cover layer can be significantly delayed or accelerated in spots, so that as a rule, it is necessary to wait for a relatively long time until uniform discoloration of the copper surface has been achieved. Deviations from uniform coloring are observed, in particular, during the initial stages of weathering. In many cases, non-uniform dark spots and/or strips form on the copper surface at first. Over the further course of weathering due to atmospheric influences, however, these color differences disappear again.
A method for the production of uniform layers of cuprous oxide on the surface of copper wire or strips is mentioned, for example, in Chemical Abstracts, Volume 83, No. 2, July 1975, page 258, Abstract No. 32184t. In this known method, the copper oxide layer is formed by means of oxidizing heat treatment at a temperature lying in the temperature range of 300 to 1000.degree. C.